1. Field of the Invention
The present invention relates to a process for the production of semi-finished articles of hard steels in which the carbon content is higher than 0.75%, said process utilizing a continuous casting operation.
The disclosed process can be used in particular but not exclusively for the production of tubular articles intended for the production of ball or roller bearing assemblies. A substantial part of a ball or roller bearing assembly is produced at the present time from a hard steel which complies with standard 100 C6 (US standard 52100) having a mean carbon content of about 1% and a mean chromium content of about 1.5%. Compositions derived therefrom can also be used, which contain trace amounts of chromium and additionally Mo, Mn or other conventional alloying elements.
2. Description of the Prior Art
Semi-finished articles such as bars, wires or tubes, which are made of such steels, are generally produced in accordance with the following conventional operation procedure:
1. preparation of said steel from, for example, pig iron in an electric furnace or converter,
2. casting in the form of ingots,
3. rolling on a blooming mill, and
4. re-rolling on a bar, wire or tube train.
When hard steels, in particular those of the above-indicated type which are hyper-eutectoid, solidify in an ingot mould, inevitable major segregation occurs accompanied by axial concentration of large primary carbide grains having acute angles thus producing grains which are difficult to dissolve and which tend to be retained in the structure of the rolled article. This is due in part to the grains possessing shapes and dimensions which are incompatible with the tempering standards applicable to such steels which normally permits dissolution of the carbides in final hardening operations. To remedy this, these steels require special hot-transformation operations in order to remove such major-segregation hypereutectoid carbides and this is normally accomplished by maintaining the ingot at a high temperature, for example from about .theta.s -75.degree. C. .theta.s is the temperature of the solidus of the steel the incipient melting point the incipient melting point. This temperature is maintained for several hours so as to cause partial dissolution of the large grains of carbide which are accumulated at the centre of the ingot. Dissolution of those carbides is completed by a rolling operation carried out by the blooming mill directly after the partial dissolution of the carbides.
Experience has shown that applying such a treatment to rough castings of smaller section, such as for example continuous casting billets, gives rise to serious disadvantages. Maintaining the metal at a high temperature during a homogenization treatment leads to large metal losses due to oxidation, and excessive decarburization, with the results that subsequent rolling operation is much less effective in eliminating the carbides. Studies that have been carried out have shown that this is because the core of the rough casting in which the primary carbides are concentrated cools excessively during the rolling operation.